The present invention relates to a wood-type golf club head having stable performances of flight direction and flight distance of hit ball.
As shown in FIG. 8A as to a wood-type golf club head “a” for right-handed golfers (all explanations made herein being for right-handed golfers), if a golf ball “b” is struck by the wood-type golf club head “a” at a position on a toe side of a sweet spot SS that is a point at which a normal line drawn from the center of gravity G of the club head with respect to a face “f” intersects the face “f” (such a hitting may be hereinafter referred to as “toe hit”), the club head rotates clockwise about the center of gravity G. In the ball which is in contact with the face, a side spin which causes the ball to rotate in the counterclockwise direction opposite to the rotation of the club head (i.e., so-called hook spin for right-handed golfers) generates. On the other hand, if the club head hits the ball on a heel side of the sweet spot SS (such a hitting may be hereinafter referred to as “heel hit”), the club head rotates counterclockwise about the center of gravity G, and the ball which is in contact with the face causes a side spin of the clockwise rotation (so-called slice spin). Such a phenomenon is well known as a gear effect. If the face “f” is flat, as shown in FIG. 8A, the ball “b” is struck out approximately parallel to the target flight direction “j” and thereafter the ball is driven to the left or right direction to cause a hook or slice.
In order to solve such a problem of poor flight direction performance, a face bulge has been conventionally provided to the face “f” of the wood-type golf club head “a”, as shown in FIG. 8B. The face bulge is a rounded or curved surface having a radius of curvature Rx which is smoothly and slightly convex toward the front side. In case of a toe hit by a club head provided with a face bulge, the ball is struck out at a deflection angle θ with respect to the target flight direction “j” by the face bulge, and then would hook by a side spin to curve back toward the target flight direction. In case of a heel hit, the ball is struck out at a deflection angle to the left of the target flight direction “j”, and then would slice by a side spin to curve back toward the target flight direction. Therefore, the face bulge serves to improve the flight direction performance by a side spin caused by a horizontal gear effect.
A similar phenomenon to the horizontal gear effect also takes place about a horizontal axis passing through the head's center of gravity G in the toe-heel direction. This may also be called “vertical gear effect”. For example, as shown in FIG. 9A, if the club head “a” hits the golf ball “b” on a crown “c” side above the sweet spot SS of the face “f” (such a hitting may be hereinafter referred to as “high hit”) or on a sole “s” side below the sweet spot SS (such a hitting may be hereinafter referred to as “low hit”), the club head rotates about the horizontal toe-heel axis by a moment which is the product of a force F received from the golf ball “b” and a vertical distance L1 or L2 between a hitting position and the center of gravity G. At that time, the golf ball contacting the face “f” receives a force acting in the direction opposite to the rotation of the club head “a” by a frictional force. Thus, off-center hits above the sweet spot reduce the amount of backspin of the golf ball “b”, and off-center hits below the sweet spot increase the amount of the backspin.
In order to prevent such an uneven backspin amount from occurring, a face roll has been conventionally provided to the face “f” of the wood-type golf club head “a”, as shown in FIG. 9A. The face roll is a rounded or curved surface having a radius of curvature Ry which is smoothly and slightly convex toward the front side when the face is viewed from the side.
In case of hitting a golf ball “b” by club heads “a” shown in FIGS. 9A and 9B at the same position below or above the sweet spot SS, the vertical distance L1 or L2 between the force F and the center of gravity G is larger when hitting by a face “fn” having no face roll of the club head shown in FIG. 9B as compared with the hitting by a face “f” having a face roll of the club head shown in FIG. 9A. In other words, the rotational moment is reduced as the radius of curvature Ry of the face roll is reduced and, therefore, the vertical gear effect can be reduced. Further, in case of off-center hits above the sweet spot, the face roll increases the launch angle δ. This is useful for preventing the flight height from lacking with decrease in the amount of backspin on off-center hits above the sweet spot. The face roll reduces the launch angle δ for off-center hits below the sweet spot and, therefore, it is also effective in preventing a hit ball from flying too high owing to increased amount of backspin.
Size increase of golf club heads has progressed rapidly with recent development of thin wall molding technology for metal materials. Large-sized golf club heads enable to have a large moment of inertia about a vertical axis passing through the center of gravity. For example, club heads having a moment of inertia of 4,000 g·cm2 or more are known. However, in case of club heads having a large moment of inertia about the vertical axis, the amount of rotation or twisting of the head “a” about the vertical axis is small for both the toe hit and the heel hit. Thus, the amount of sidespin imparted to the ball by the horizontal gear effect is also small. Therefore, as shown in FIG. 8C, club heads having, for example, a small face bulge radius Rx (i.e., a large curvature) and a large moment of inertia about the vertical axis cannot impart a sidespin in an amount commensurate with the deflection angle θ of the hit ball, in the toe hits or the heel hits, despite that the deflection angle θ becomes large. Since the hit ball does not curve back to the target flight direction “j” by such a reason, large-sized club heads have a problem of poor flight direction performance.
JP-A-2001-161866 discloses a golf club head having a horizontal bulge radius R1 of 480 to 765 mm and a vertical roll radius R2 larger than the radius R1. JP-A-8-089603 disclose a golf club head having a horizontal bulge radius R1 of at most 9 inches and a vertical roll radius R2 of at most 9 inches. In these prior art, the horizontal bulge radius and the vertical roll radius are not specified in association with the moment of inertia about the vertical axis of the head. Thus, these proposed golf club heads still have room for improvement.
It is an object of the present invention to provide a wood-type golf club head having an improved directional stability of hit ball and an increased flight distance.
This and other objects of the present invention will become apparent from the description hereinafter.